When Firefox decided to change the shell of the browser and move to WebExtensions (same format of extensions as used by Chrome) you have not been able to change the interface as you wished anymore via browser extension.

I was using TabMix Plus, and later one Tree Style Tab extensions in order to fit many open tabs and have them all visible at the same time. Before, extensions could automatically disable tab bar for example and minimize vertical space usage.

As you can’t do that anymore the only thing left to do is to change your chrome CSS manually:

Go to the profile directory, on Linux it is:

cd ~/.mozzila/firefox/YOUR_PROFILE/

If chrome directory doesn’t exist, create it:

mkdir chrome

Create file called userChrome.css and add following content:

TabsToolbar { visibility: collapse !important; }

Save the file and restart the web browser